1. Field of the Invention
The invention relates to electron energy loss spectrometers, and particularly to high resolution electron energy loss spectrometers used with transmission electron microscopes.
2. Description of Prior Art
When a beam of medium energy (10 keV to 10 MeV) electrons traverses a thin sample, the electrons loose various amounts of energy which are characteristic of the sample. If the beam is subsequently dispersed into an electron energy loss spectrum, much useful information can be learnt about the sample chemical composition, chemical bonding, and structural, electronic and vibrational properties.
The actual amount of information present in the energy loss spectrum depends on the spectrum's energy resolution. Information on the sample chemistry is present in spectra of about 3 eV resolution, information on the chemical bonding requires about 0.5 eV resolution, information on the sample electronic properties requires about 0.1 eV resolution, and information on the sample vibrational properties is typically only present in spectra which show a resolution of a few meV (millielectron Volts). Consequently, there is much interest in improving the energy resolution of electron energy loss spectrometers. There is also considerable interest in improving the spatial resolution of energy loss spectroscopy so that very small sample areas can be analyzed selectively. High spatial resolution is achieved by combining energy loss spectrometers with bright electron guns and probe-forming lenses that are able to focus an electron beam into a probe of 1 nm or less in diameter. Because transmission electron microscopes typically comprise both bright electron guns and high-performance probe-forming lenses, there is much interest in incorporating high resolution electron energy loss spectrometers into the columns of transmission electron microscopes.
The energy resolution of electron spectrometer systems is limited by several parameters. The chief among these are the energy spread of the electron gun, and the instabilities of the high voltage power supply of the electron gun and of the power supplies of the energy analyzer(s). The energy spread of the electron gun is typically about 1 eV and rarely less than 0.2 eV. In order to attain an energy resolution better than 0.2 eV, the energy spread of the electron beam therefore has to be decreased by a monochromator before the beam reaches the sample. The instabilities of the high voltage power supply of the electron gun are typically about 0.5 V and rarely less than 0.1 V, and the instabilities of the power supplies of the spectrometer are typically comparable in their effect on the energy resolution. If the spectrometer system is not equipped with a means for canceling the influence of the instabilities on the resolution, its energy resolution is therefore limited to worse than 0.1 eV even when monochromation is employed.
An electron energy loss spectrometer using a thermionic electron gun of 30 keV primary energy, and crossed E/B field energy analyzers (of the type known as a Wien filter) for both monochromating the electron beam and producing the energy loss spectrum was designed and built by Boersch and later improved by Geiger and co-workers. This apparatus deaccelerated the electron beam to an energy of a few eV before passing it through the monochromator, reaccelerated the beam back to 30 kV before the beam traversed the sample, and then deaccelerated and reaccelerated the beam before and after passing it through the second energy analyzer. The high voltage for the electron gun and all the deaccelerating and accelerating regions came from the same power supply, thus assuring that fluctuations of even several volts in the high voltage would not affect the energy resolution of the total apparatus, which was able to deliver an energy resolution of about 3 meV. However, the need for two deaccelerating and two accelerating regions makes Boersch's design unsuitable for primary voltages higher than about 100 kV, which are desirable in an electron spectrometer system. Further, Boersch's apparatus had very poor spatial resolution. It produced spectra from mm-sized regions of the sample, whereas it would be highly desirable to be able to produce spectra from nm-sized regions.
A different type of an electron energy loss spectrometer apparatus capable of giving an energy resolution better than 0.1 eV was designed and built by Schnatterly and co-workers. This apparatus monochromated the electron beam while it was at a low energy, then accelerated the beam to a voltage of 100 kV to 200 kV, passed the beam through a sample, and finally deaccelerated the beam to a few eV before analyzing it. The gun, the monochromator, and the final energy analyzer were all held close to ground voltage, while the sample and the rest of the optical column were held at a high voltage. Schnatterly's design had the advantage of relative simplicity compared to Boersch's design, but the major disadvantage that the electron-optical components which formed the electron probe incident on the sample were all held at high voltage. This precluded the use of magnetic lenses, with the result that the smallest attainable probe size was about 0.5 mm, while an apparatus using magnetic lenses to focus 100 kV to 200 kV electrons into a small spot can easily attain a probe size of a few nm. A further major disadvantage of the Schnatterly apparatus was that because the sample was held at the high voltage, access to the sample during operation was not available. Therefore, it was not possible to do any of the operations which are customary in most electron microscopes, such as shifting the sample to select a desirable specimen area, tilting the sample to choose its orientation with respect to the electron beam, heating or cooling the sample, etc.
Most users of electron spectrometers would find significant advantage in an electron energy loss spectrometer system which combined good spatial resolution with an energy resolution of a few meV, and which was capable of operating at primary energies as high as several hundred keV or even several MeV. They would also find advantage in an electron spectrometer system which avoided the use of high voltage in places other than the electron gun, and was able to tolerate substantial instabilities of the gun high voltage power supply and of other critical power supplies such as those of the energy analyzers, without these instabilities resulting in a deterioration of the energy resolution of the total system. They would further find advantage in an apparatus which did not hold the sample at a high voltage, but to the contrary held the sample at or close to ground voltage, thereby facilitating access to the sample during an experiment.